I caught several
examples of this species of jellyfish in the nearby sound — they're apparently
quite prolific. And they also appear to be harmless to humans, for which I'm
thankful. They're so transparent as to be virtually invisible even from very
short distances, and on several occasions I discovered one when it was only
centimeters from my dive mask. And abruptly, I would realize that I was in the
water with quite a few of them, smack in the middle of the smack, as it were
('smack' is the term for a collection of jellyfish, like 'herd' or 'pride' or
'gaggle' — didn't know that, did you?). Contact was impossible to avoid, but
after several such accidents, I began ignoring them to no ill effect.

According to what
I can find, this is a variety of Comb Jellyfish (phylum Ctenophora,) about the size of a golf ball.
I'm not even going to attempt to describe the actual body shape (and it's questionable
with jellyfish anyway), but the lines along the body are rows of tiny fins for
maneuvering. The motion of the fins is so rapid, in contrast with the painfully
slow movement of the jellyfish through the water, that you almost expect to
hear the sounds of a blimp. Moreover, the fins refract the light in the right
conditions, producing rainbow ripples that run down the body, an effect I was
lucky enough to capture with this photo — it took enough attempts, believe
me.

Official accounts
have called this species bioluminescent, but this isn't something I've witnessed
for myself. The rainbow colors only appear in bright light, and vanish as soon
as the light is extinguished. I observed one specimen in a pitch dark room for
several minutes and witnessed nothing, except for the bioluminescent plankton
that shared the tank at the time.

I couldn't keep any of
these in my tank, since the circulator overpowered their weak ability to swim
and they would quickly end up pinned against the intake, which will injure them after
a while. This shot was taken aiming straight down into the tank when the jellyfish
was right at the surface, with the strobe aimed directly from the side below
the waterline. The 28-105 zoom allowed me the right working distance and framing,
and an aperture of f22 provided some leeway in tight focusing, difficult to
do with a handheld camera at macro distances.